Irrigation control system

ABSTRACT

Irrigation control apparatus includes: (a) a mast having a lower portion adapted to be removably inserted into the earth, (b) soil moisture sensing structure associated with or separate from said lower portion of the mast, and (c) data transmitting apparatus on an upper portion of the mast and operatively connected with said sensing structure for transmitting to a central station data indicative of soil moisture conditions at the location of the mast.

BACKGROUND OF THE INVENTION

This application is a continuation-in-part of application Serial No. 221,519, filed Dec. 30, 1980, now Patent No. 4,396,149.

This invention relates generally to irrigation management, and more particularly concerns a system which conserves water and energy by providing optional or highly efficient irrigation to growing plants.

The most basic form of present irrigation control involves a decision by a human to begin or discontinue application of irrigation water. The effects of under-irrigation are obvious in that plant condition suffers; however, in many cases over irrigation does not cause apparent harm to the plants. In such a case, the major harm caused by over-irrigation is the waste of irrigation water and the energy required for it distribution. Additionally, however, there may be subtle but significant impairment of plant growth.

Several methods have in the past been developed to improve the efficiency of irrigation and at the same time minimize the burden on human operators. One technique is the use of preprogrammed timers to automatically start and terminate irrigation. Such timer systems can be enhanced to provide for mutiple timing cycles which vary in accordance with plant growth cycles. However, such systems have the limitation that there is no feedback between actual plant water consumption and water application. For example, timer systems are unable to reduce water application during times of humid or rainy weather, and do not account for variable water requirements.

Because of these limitations, systems which measure free soil moisture were devised. Such systems measure soil moisture in a variety of ways, including soil electrical conductivity, and by use of tensiometers. Typically, water application is commenced when the soil moisture is diminished to a present level; and irrigation discontinued when the moisture again rises to a predetermined level. Such operation has several limitations, relating primarily to the time delay between the application of irrigation water, and the measurement of changes in the soil moisture at depth. For example, the moisture sensors at depth may still be wet when shallow plant roots are already becoming dry. Similarly, the irrigation water may not have reached the sensors even when the surface has been flooded with water.

Efforts to overcome these problems have involved the combining of timers with soil moisture sensors. In such systems, the timers actuate irrigation at predetermined times, but only if the moisture soil sensors indicate that the soil has become dry. The timers eliminate the danger of overwatering while waiting for moisture to penetrate to the sensors. Still, they are not responsive to changes in weather, nor can they sense any characteristics other than the simple value of the soil moisture.

Another broad aspect which has been addressed previously is the concept of using telemetry to acquire data from the agricultural field, and to use data to obtained to actuate irrigation controllers. Systems described heretofore, however, lack sufficient control.

SUMMARY OF THE INVENTION

It is a major object of the present invention to provide an irrigation control system, and components of such a system, which overcome the deficiencies of prior control systems and efforts, and to provide optimal irrigation to growing plants.

Viewed in its system aspects, the present invention incorporates:

1. Multiple sensors measuring soil moisture at various appropriate depths and sites in a field or fields;

2. Weather measurement equipment associated with the field or fields;

3. Apparatus and method to telemeter soil moisture and weather data from measurement sites to a central computer;

4. Computer means equipped to receive and transmit data, and programmed with instructions and data which compute optimal irrigation strategy;

5. Apparatus and method to display the measured data and the computed irrigation strategy to a human operator;

6. Apparatus and method to telemeter actuation commands from the computer to irrigation controls; and

7. Apparatus and method to control the flow of irrigation water as directed by the telemetered commands.

More specifically, the soil measurement sensors, weather measurement equipment and telemetering apparatus may be incorporated on multiple stations spaced apart in a field, each station including a mast, the lower end of which is adapted to be removably inserted into the earth to expose the sensors to soil moisture conditions, or to be used in conjunction with separate sensors; and the computer means may include a field computer unit and associated telemetry devices. This field computer unit is programmed with instructions and data to compute optimal irrigation, and it is also equipped with data terminals which allow it to communicate with humans, such as the farmer.

These and other objects and advantages of the invention, as well as the details of an illustrative embodiment, will be more fully understood from the following description and drawings, in which:

DRAWING DESCRIPTION

FIG. 1 is a schematic view of a system incorporating the invention;

FIG. 2 is a schematic view of a typical farm incorporating the invention;

FIG. 3 is an elevational view of a soil sensor station as incorporated in FIG. 2;

FIG. 3a is an elevation showing a modified sensor station;

FIG. 3b is a fragmentary view of a bendable support at a soil sensor station;

FIG. 4 is a block diagram showing electrical elements associated with the FIG. 3 station;

FIG. 5 is a block diagram showing electrical elements associated with the weather station illustrated in FIG. 2;

FIG. 6 is a block diagram showing electrical components incorporating in the pump control and monitor station illustrated in FIG. 2;

FIG. 7 is a block diagram showing a remote computer station;

FIG. 8 is a predictive model circuit diagram implemented in the FIG. 7 remote computer;

FIG. 9 is a perspective view of a signal generation means to develop signals as a function of consumptive use of water and desired moisture level.

DETAILED DESCRIPTION

Referring first to FIG. 1, farms or fields are shown at 10, with a remote central computer at 11. Remote computers 12 are associated with the farms. The central computer 11 serves to provide backup data storage; and to provide updated information to the remote computers. Transmission between the remote computers and the central computer is via ordinary telephone lines, indicated at 13. Each remote computer can handle many separate fields at a farm.

In FIG. 2 a typical farm 15 includes several fields 15a. Associated with each farm 15 is a remote computer 16 which is able to communicate via radio or other means with various sensor and control units 17 and 18 located throughout the farm or fields. Each field is typically instrumented with several soil sensor stations or units; in FIG. 2, three such soil sensor stations are shown in one field and spaced along and between crop rows 200. In actual practice, many fields could be instrumented, and with as many sensor stations as are needed to characterize that field, i.e. to develop soil moisture data characteristic of the field.

Additionally, a weather station 19 is provided and may be located in any field. This station supplies data including, but not limited to: temperature, humidity, wind velocity, wind direction, isolation (sun intensity), and rainfall amount. Antennas associated with the described elements are indicated at 16a, 17a, 18a and 19a.

Also, the system is able to control irrigation (start and stop pumps, open and close valves, etc.) for those farms where such facilities are available. See for example pump or valve 20 connected at 21 with controller 18. The pump or valve typically controls water flow to one or more of the fields 15a.

There is also means (for example telephone or radio 22, and link 23) for the system to communicate with the farmer or other human. This capability allows the farmer to obtain reports on the system's operation, to manually overide the system's activities (for example, to inhibit irrigation in a field where harvesting is planned); and for those farms where automatic control of pumps or valves is unavailable, to advise the farmer of recommended irrigation schedules.

FIG. 3 shows a typical portable soil sensor station 17. It comprises a self-contained unit which is placed in the field, and operates to telemeter data indicative of the moisture level in the soil in the vicinity of the active region of the crop roots at that portion of the field where the station is located.

The station is designed so that it can be easily installed in the field, and easily removed, so as to facilitate harvesting and field preparation. The support shaft or mast 26 has a lower portion 26a incorporating or carrying the moisture sensitive elements 27a of the soil sensors 27. Optionally, the moisture sensor elements 27a can be mounted separate from support 26.

The soil sensor, such as tensiometer elements, are placed in proximity with the active areas of the plant roots 28 as by implanting the lower portion 26a underground, i.e. below ground level 29. (Sensors other than tensiometers are usable). Note tapered lower portion 26b of that mast, easily forced into the earth. Markers 30 on the mast indicate depth of soil penetration. The units 17 may be placed along rows of planted crops, and the construction described above allows the sensors to take up a minimum of space along the rows, with minimum interference with farm operations.

Alternatively, the soil sensors can be placed, separately or in common, on support shafts and connected to the remainder of the unit by wires or other means. This allows separation among the soil sensor elements. See for example sensors 227 in FIG. 3a, separate from mast 26 but connected by wires 228 with cable 204 in the mast (shown in FIG. 3b) as via circuitry in case 225. The sensors are located at the lower ends of plastic pipes 230 filled with sand to assure continued contact between the sensor tips and the surrounding soil. Pipes 230 are of different lengths.

The support shaft or mast 26 also has an upper or above ground portion 26c which provides a mounting means for: a radio antenna 17a or other means of telemetry; a protective case 32 which contains electrical components, such as transmitters, receivers; etc; and a solar panel 33 to provide electrical power to the electrical devices at the station 17. Alternatively, the protective case may be mounted below ground, or may be detached from support shaft 26. See case 225 in FIG. 3a, for example.

In order to facilitate farming operations, a means is provided which allows the supporting shaft to bend laterally and downwardly, so that farming operations, such as cultivation or plowing, could be conducted with the sensing unit still in place. Such means may take the form of a coil spring 202 attached to the mast sections 26d and 26e in FIG. 3b, near ground level. If the mast upper section is struck by a farm vehicle, it bends from upright (see axis 203) to a lateral and downward position (see axis 203a). The data transmitting cable 204 in the tubular mast also bends.

Examples of usable soil sensors are tensiometers described in U.S. Patent 3,224,676, soil moisture samplers as described in U.S. Patent 4,197,866, or other appropriate devices. Their vertical spacing enables ascertainment of soil moisture data at two or more depths, for establishing a moisture gradient.

FIG. 4 illustrates the electrical components of a typical sensor station 17.

Power developed at the solar panel 33 is delivered at 34 to a voltage regulator circuit 35 and used to charge a rechargable battery 36. The system typically provides power without wires, and allows power availability during times without sunlight. Power from the rechargable battery is supplied at 37 to all other states of the unit.

Alternatively, especially in the case of units associated with pumps or valves, power can be obtained from the power mains, with or without benefit of a rechargable battery used to supply power in case of power mains interruptions. See mains 205, voltage regulator 206, and protective rectifier 207.

Commands from the remote computer 16 are received via antenna 17a and receiver-amplifier 38, and then sent at 39 to a controller 40. The latter verifies that the commands were addressed to the particular sensor station; and determines the measurement desired. When an appropriate activating command is received, the signals from the soil moisture sensors, as sensed by transducers 41, are used to modulate the transmitted waveform. For example, the voltage outputs of transducers 41 modulate (at modulator 42) the waveform transmitted via transmitter 43). Thus, the signals are telemetered back to the remote computer 16 upon command.

It is also a desirable characteristic of the system that the controller can be directed to collect soil data automatically, and to report the collected data to the remote computer at different or less frequent times.

FIG. 5 shows a representation of the circuitry in the weather station 19. Data including, but not limited to, the following are collected by conventional instruments 44-49; temperature, windspeed, wind direction, humidity, insolation, and rainfall. These instruments, together with encoder 50, transmitter 51 and receiver 52 components are carried by a mast or shaft 53 (like shaft 26) and adapted to be removably implanted in the soil, or mounted in any other convenient manner.

The remote computer 16 addresses the weather station through antenna 19a and the radio receiver 52, or other communication system as may be provided. Upon receipt of such a command, an enabling signal is transmitted at 54 to the encoder 50, enabling it to encode the collected weather data (fed to the encoder at 55), and transmit same to the remote computer (via transmitter 51 and antenna 19a).

FIG. 6 shows a typical pump control/monitor station 18. Its purpose is to allow the remote computer to command the pumps to start or stop; and to allow data concerning the pump's operation to be transmitted to the remote computer.

Transmitted commands from the remote computer 16 are received via antenna 18a and receiver 60, and processed by a controller 61. Depending on the particular command, the controller can either control the pump; or it can collect data about the pump's operation. If the command is to control the pump, then controller 61 enables at 62 a power controller 63, which turns the power to the pump 20 on or off as appropriate. A power meter appears at 64.

Various parameters concerning the operation of the pump are encoded and stored. Such parameters include, but are not limited to power consumption, flow, current consumption, pumping depth and status of hand operated controls. In addition, means may be provided by which a person at the pump site can communicate with the computer and influence control.

For example, the energy consumption or the water flow is encoded at 65 and telemetered at 66. Data from water meter 67 is transmitted at 68 to the encoder, and data from the power meter 64 is transmitted at 69 to the encoder.

The physical construction of the pump control/monitor station is similar to the soil sensor station shown in FIG. 3 i.e. the components 18a, and 60-69 may be carried by a mast or shaft 70 removably implanted in the soil, adjacent pump 20, or mounted in other convenient manner.

FIG. 7 shows the remote computer 16, which collects data from the fields, makes decision about optimal irrigation strategy, communicates with the central computer, and communicates with the farmer. The internal characteristics of the remote computer are illustrated in the following figures.

The remote computer is able to communicate with the central computer using a modem (modulator-demodulator) 71 of conventional design, such as a Bell type 103. In turn, communication is through existing telephone network 13.

Control/display function at 72 is provided using potentiometers, switches, dials, which indicate the status of the internal circuits and allow control thereof. A radio transmitter/receiver or other means of communication with the sensors and controllers in the field is provided by antenna 16a, receiver 74 and transmitter 75. In operation, data is collected in the computer memory from the soil sensors which indicates the free moisture in various portions of each field. Weather data is collected from the weather station. Means is provided to supply data to the computer indicative of the consumptive use characteristics of the crop (the variations in water consumption throughout the growing seasons). Characteristics such as soil type, irrigation flow rate, etc. are thereby applied. The farmer can provide additional information, such as "water now", "do not water", etc.

Using all the above factors, the remote computer develops a predictive model of the amount of soil moisture in each field; and predicts the optimal irrigation strategy. In this regard it is distinctive, and is predictive, rather than reactive.

The predictive model implemented in the remote computer can be implemented in many ways, and FIG. 8 shows one possible implementation using analog electronic circuits. Voltages corresponding to temperature, insolation (sun intensity) and humidity are derived from data collected by the weather station. These voltages supplied on lines 76-78 are summed at 79 with appropriate weighting. The humidity, in this case, would have a negative weight, represented by the invertor 80 in the figure. The exact weighting factors would vary from case to case, and can be varied as by controls 76a, 77a and 78a. The weighted summing can be implemented in several ways, one of which would be conventional operational amplifiers. The result of this weighted summing would be a voltage (transmitted at 81) which is an analog of the overall weather situation, hotter, dryer weather would in general be represented by a greater voltage. This signal is also optionally displayed at 82 to the human operator. It is labelled "weather factor" in the figure.

Another input to the system is a voltage (supplied at 84) which is an analog of the soil characteristics. Since this factor would be constant over extended periods of time, it may be set by a potentiometer 85. In the particular model shown, a higher voltage would be analogous to sandy soil, in which water was rapidly lost through downward percolation. A lower voltage would represent clayey soil which tended to retain water.

A third input at 86 is a voltage analogous to the consumptive use of the crop. This is labelled "consumptive use factor" in the figure. It corresponds to the rate at which water is consumed by the plants themselves. A greater voltage represents greater water consumption, such as during a very active growing phase. The consumptive use factor depends on the crop type, location, phase of the growing season, etc. FIG. 9 shows one means for developing this voltage analog.

The three factors described above-weather factor, soil factor, and consumptive use factor-are in turn processed by a weighted summing circuit 90. The output 91 of this circuit, labelled "water use factor" is an analog of the overall water lost from the soil for all reasons in the model.

A voltage is stored on capacitor C. This voltage appearing at 92 is labelled "modelled soil moisture", and is an analog of the estimate of the amount of water stored in the soil. The water use factor is used to control a voltage controlled resistor 93. A greater water use factor would cause the resistance to decrease. Such a decrease would cause capacitor C to discharge at a greater rate. The discharge current at 94 is labelled "water loss" in the figure. Thus, the foregoing is a model of the depletion of water from the soil.

In the particular model shown in the figure, water can be added to the soil from two sources: rainfall and irrigation.

A voltage analogous of the rate of rainfall is developed at 95 from data collected from the weather stations. This is labelled "rainfall rate" in the figure. No rainfall is represented by zero volts, and greater rainfall is represented by greater voltages. A current proportional to the rate of rainfall at 96 through the variable resistor 97 labelled rainfall scale factor, and charges the capacitor C. This causes an increase in the modelled soil moisture. The imedance of the rainfall scale factor resistor is determined by the particular location, and factors such as the portion of rainfall which is absorbed by the soil, rather than drained away and lost, etc.

A second method by which the modelled soil moisture can be increased is if irrigation is performed. Thus, if the model issues an "irrigate" command (in a manner to be described below) then relay Y is energized. Current flows from source 100 through the variable resistor 98 labelled "irrigation rate factor", and charges capacitor C. This, in turn, increases the modelled soil moisture. The irrigation rate factor resistor is adjusted to represent the particular installation; for example, large pumps would be modelled by a small value of the resistor, and corresponding large currents.

The result of the foregoing is the modelled soil moisture. It is an estimate of the water stored in the soil, and available to the plants. However, the model described so far has no provision for adjusting the model to approximate the actual measured values of soil moisture. The following is one means to accomplish this correction, and prevent the model from becoming grossly different from actuality.

Voltages analogous to the measured soil moisture levels are developed. These are based on the data obtained from the soil sensor stations described previously. They are labelled "soil moisture levels" 101 in the figure; higher voltages represent greater measured moistures. These voltages are applied to a circuit 102 which produces as its output 103 a voltage analogous to the overall moisture level in the field and which is a function of levels 101. The exact functional relationship between the several soil moisture levels, and the representative single value, labelled "present water factor" in the figure could be any one of many. For example, on possibility would be for the present water factor to be the simple average of the several soil moisture levels. Another possibility might be to use the lowest of the soil moisture levels, thus basing actions on the dryest. Another might be to use the tenth percentile, or other means. In any case, a single voltage analogous of the overall actual condition of soil moisture is developed at 103.

The modelled soil moisture signal 92 is then compared with the actual overall soil moisture (signal 103) in a differencing circuit 104. The output 105 of this differencing circuit is a correction factor which represent error in the model when compared to actuality.

This correction factor is passed through a low pass circuit 106 which eliminates short term difference, and develops a signal 107 labelled "long term averaged difference". This voltage is analogous to the long term difference between the modelled soil moisture and the observed soil moisture. The long term averaged difference signal 107 is summed in summing circuit 109 with the modelled soil moisture signal 42 to produce a signal 110 labelled "corrected modelled soil moisture" in the figure. This is the model's best estimate of the soil moisture.

Another input to the system is a voltage 111 labelled "desired soil moisture level" in the figure. This could be set by a potentiometer at 112 if it remains relatively constant, or could be generated by a device such as is shown in FIG. 9. This is analogous to the desired level of moisture in the soil; and depends on the crop type, location and other factors; and may in some cases vary through the growing season.

The desired soil moisture can also be set to a very high value or a very low value, using the switch labelled "override control" 113. In any case the desired soil moisture is compared with the corrected modelled soil moisture, in the comparator 114. If the desired moisture is less than the corrected model, then the output 115 of the comparator goes high, and irrigation is commenced via pump controls 61 and 62. A display 116 also indicates the beginning of irrigation, and relay Y is energized, so that the modelled soil moisture can be adjusted to reflect the beginning of irrigation. When the corrected model of soil moisture reaches the level of the desired moisture, the output of the comparator goes low, a command to discontinue irrigation is issued, and relay Y is deenergized.

Thus, the model in the figure and described here is one means to provide a predictive model of soil moisture, and an optimal means of irrigation control.

Certain signals used by the model described previously may vary throughout the growing season. These include the consumptive use of water by plants; and also the desired soil moisture levels. FIG. 9 shows one means to develop these signals.

A narrow, parallel beam of light 120 is generated. One known way to do this would be with a lens system and shutters. This beam of light is directed through a strip of photographic film 121 of other material which has been prepared with a clear (transparent) region 122 and an opague region 123. The strip of film is slowly moved from a supply spool 124 to a take-up spool 125 during the course of the growing season. At any time in the season, the portion of the film through which the light shines has a transparent area which is analogous to the value of the appropriate signal (consumptive use) at that time.

The light which then passes through the strip is proportional to the portion of the strip which is transparent. The light is concentrated by a lens 126 and is used to illuminate a photocell 127. Thus, the voltaged developed by the photocell is representative of the desired signal. A very slow speed drive for the take-up spool is indicated at 128.

Representative commercially available components indicated in blocks in the above description, are listed as follows:

    ______________________________________                                         Components      Model and Manufacturer                                         ______________________________________                                         11              Digital Equipment Corp.                                                        PDP-11/70 Computer                                             16              Digital Equipment Corp.                                                        PDP-11/03 Computer                                             20              Rainbird EP-300-F                                              27              Irrometer                                                      33              Photowatt International                                                        MA 2320                                                        35              Motorola MC7812                                                38              REPCO 810-055                                                  40              Energy Management 8120                                         41              Energy Management 8120                                         42              Creative Micro Systems                                                         9651-509                                                       43              REPCO 810-038                                                  44              Weather Measure                                                                T 621                                                          45              Weather Measure                                                                W 121                                                          46              Weather Measure                                                                W 121                                                          47              Weather Measure                                                                H 352                                                          48              Weather Measure R 413                                          49              Weather Measure                                                                P532                                                           50              Creative Micro Systems                                                         9651-513                                                       51              REPCO 810-038                                                  52              REPCO 810-055                                                  60              REPCO 810-055                                                  61              Creative Micro Systems                                                         9651-505                                                       63              Westinghouse BF-02-F                                           64              General Electric DSW 43                                        65              Creative Micro Systems                                                         9651-513                                                       66              REPCO 810-038                                                  67              Brooks Instruments 10                                          71              RACAL-VADIC VA 305                                             72              Teletype 43                                                    74              REPCO 810-055                                                  75              REPCO 810-038                                                  79              National Semiconductor                                                         LM 324                                                         80              National Semiconductor                                                         LM 324                                                         90              National Semiconductor                                                         LM 324                                                         102             National Semiconductor                                                         LM 324                                                         104             National Semiconductor                                                         LM 324                                                         106             National Semiconductor                                                         AF-150                                                         109             National Semiconductor                                                         LM 324                                                         114             National Semiconductor                                                         LM 311                                                         ______________________________________                                     

I claim:
 1. In irrigation control apparatus, the combination comprising(a) a mast having a lower portion adapted to be removably inserted into the earth, and an upper portion operatively connected with said lower portion and adapted to be deflected laterally when struck, (b) soil moisture sensing means to be inserted into the earth to controlled depth to sense moisture at said depth, (c) and data transmitting means on an upper portion of the mast and operatively connected with said sensing means for transmitting to a central station substantially continous measurement data corresponding to moisture levels continuously sensed by said (b) means at the location of the mast.
 2. The apparatus of claim 1 wherein said sensing means comprises a sensor carried by said lower portion of the mast and connected to said (c) means.
 3. The apparatus of claim 1 wherein said sensing means includes vertically spaced sensor carried by said lower portion of the mast for establishing moisture gradient data.
 4. The apparatus of claim 1 including a multiplicity of said masts with said (b) and (c) elements associated therewith, the lower portions said masts inserted into the earth at selectively spaced locations in a field, there being bendable means interconnecting the upper and lower portions of each mast to accomodate said deflection of said mast upper portion.
 5. The apparatus of claim 4 including a computer at said central station operatively connected to said multiple data transmitting means associated with said masts, via multiple data transmission links.
 6. The apparatus of claim 1 including a computer at said central station operatively connected to the data transmitting means via a data transmission link.
 7. The combination of claim 6 including an irrigation means operatively connected with said computer to commence irrigation in response to signals transmitted from said computer as a function of data transmitted from said data transmission means.
 8. The combination of claim 7 wherein said irrigation means includes a motor driven pump to pump irrigation water.
 9. The combination of claim 7 including a data transmission link between said irrigation means and said computer to transmit data from the computer to said irrigation means, and to transmit data concerning the status of said irrigation means to the computer.
 10. The combination of claim 9 wherein said link is a radio link that incorporates a modem.
 11. The apparatus of claim 7 wherein said computer includes circuit means having inputs for data representative of actual soil moisture level, and of modulated desired moisture level, for producing a control output which is a function of a comparison of said actual and modulated desired levels, said computer operatively connected with said irrigation means so that said control output controls said irrigation means.
 12. The apparatus of claim 11 wherein said computer also includes summing circuitry having inputs for data representative of temperature, sun intensity, humidity, soil characteristic and consumptive use of water by plants, thereby to derive a summed output, and other circuitry modulated by said summed output and also by data representative of actual soil moisture level for producing an output signal representing said desired soil moisture level.
 13. The apparatus of claim 1 including a solar collector associated with said mast characterized as producing electrical energy used to power said data transmitting means.
 14. The apparatus of claim 13 including a battery associated with said mast and connected with said collector to store said electrical energy.
 15. The apparatus of claim 1 including a spring intercommunicating the mast upper and lower portions to accommodate said deflection.
 16. The apparatus of claim 1 including a protective case for circuitry connected with said data transmission means.
 17. The apparatus of claim 16 wherein said case is carried on the mast.
 18. The apparatus of claim 16 wherein said case is located proximate the mast, but spaced therefrom.
 19. The apparatus of claim 1 wherein said sensing means includes sensors carried by supports spaced from the mast.
 20. In the method of controlling irrigation of plants in a field, and employing a unit including a mast having a lower portion adapted for removable insertion into the earth, and soil moisture sensing means, the mast having an upper portion and circuit means on said upper portion operatively connected with said sensing means for substantially continuously transmitting to a control station data indicative of actual soil moisture conditions of the soil at the mast local, the steps that include:(a) inserting said mast lower portion into the earth along a plant row, and placing said sensing means below ground surface level, (b) operating said sensing means to produce said data, (c) operating said circuit means to substantially continuously transmit said data to said control station during a selected time interval, (d) processing said data to derive an output which is a function of actual soil moisture level and desired soil moisture level, (e) using said output to control irrigation of said plants in the field, (f) and allowing lateral deflection of the mast upper portion relative to said sensing means.
 21. The method of claim 20 wherein multiple of said units are employed, the additional steps that include:(g) selectively locating said units in different zones of said field, (h) maintaining data communication between said units and said control station, so that an output as in (d) is derived for each unit, (i) providing irrigation means, and (j) said (e) step including using each of said outputs to control said irrigation means.
 22. The method of claim 21 wherein said irrigation means comprise water flow control devices, and wherein said outputs are used to control the respective devices.
 23. The method of claim 20 wherein said control station includes computer means incorporating first circuit means having inputs for said data representative of actual soil moisture level as sensed by said sensing means, and of modulated desired soil moisture level, for producing said output which is a function of a comparison of said actual and modulated desired levels, and including the step of operating said computer means to produce said control output which is then used to control irrigation as in step (e) of claim
 15. 24. The method of claim 23 including the step of providing said modulated desired soil moisture level by providing input signals corresponding to temperature, sun intensity, humidity, soil characteristic, and consumptive use of water by the plants, at said field, summing said input signals to produce a summed output, and using said summed output to modulate said data representative of actual soil moisture level.
 25. The method of claim 20 including positively deflecting said upper portion of the mast, and allowing said upper portion to restore to original position.
 26. The method of claim 25 including, preliminarily, employing a spring to interconnect said mast upper and lower portions.
 27. The method of claim 20 wherein said (a) step placement of the sensing means is carried out to locate said sensing means in spaced relation to the mast.
 28. The method of claim 20 wherein said (a) step placement of the sensing means is carried out by said mast insertion step. 